1. Field of the Invention
The present invention relates to a developing device that is used in an image forming apparatus.
2. Description of the Related Art
A copying machine is much used as an image forming apparatus. In the copying machine, a developing device is used for developing an electrostatic latent image formed on an image carrier to obtain a toner image on an image receiving medium.
Of the developing devices, a two-component developing device using a developer comprising toner and carrier is also much used for color recording machines and printers from the viewpoint of image quality. In the color recording, a ratio of the image area portion is higher than the conventional monochrome recording because of image feature and toner consumption is greater by about 5 times.
Therefore, in the color recording, more toner must be supplied in a relatively short period and furthermore, a charging characteristic sufficient enough for development must be given to the toner before supplying it to the developing roller.
On the other hand, the downsizing and speed increasing of color image forming apparatuses is being progressively demanded.
In order to give a sufficient charging characteristic to toner that is supplied to the developing device, it is required to sufficiently stir and mix toner with carrier and a time required therefor, and the developing device itself must have a sufficient capacity to retain developer.
To secure the developer capacity and a stirring time described above, a means is adopted to arrange mixers in a developing device in a vertical direction instead of the conventional lateral side by side arrangement. Such a developing device with the mixers arranged in the vertical direction is called an HL type developing device.
The conventional HL type developing device has a partition wall built in the main body of the device for housing two-component developer to divide the inside of the main body into the upper and lower portions. An upper mixer is provided in the upper portion divided by the partition wall and a lower mixer is provided in the lower portion. There are specified gaps between both ends of the partition wall in the longitudinal direction and the side walls of the main body of the developing device so that developer is able to pass through the gaps freely.
At the front side of the partition wall and the upper and lower mixers, a developing roller, which is a developer carrying member, is provided.
In such a developing device, the developer that is stirred and conveyed by the upper mixer is conveyed in the specified direction and supplied to the developing roller as if falling thereto.
Developer that is not yet reaching the developing roller and excess developer remaining the developing roller after completing the development fall to the lower mixer side by gravity and then, is conveyed by the action of the lower mixer in the direction reverse to the upper mixer.
The developer conveyed by this lower mixer is again supplied to the upper mixer and is circulated and conveyed. In this case, however, as developer is conveyed from the lower mixer to the upper mixer against its gravity, there is such a problem that developer is not sufficiently supplied. To cope with this problem, a drawing up roller made of a magnet is provided at the end of the developing roller to convey developer magnetically. However, when developer is conveyed magnetically, there is still a problem.
That is, from the viewpoint of the stirring and conveying efficiency of developer in the developing device, it is preferred to separate developer from the drawing up roller completely.
However, at the end of using developer, developer was not completely separated from the drawing up roller for deterioration of fluidity of developer, change in characteristic of developer resulting from environmental change and part of developer was kept adhered to the drawing up roller.
If such a phenomenon is generated, the drawing up efficiency becomes worse and the flow of developer becomes slow throughout the entire developing device. If the flow of developer becomes slower than a certain level of speed, toner density becomes uneven, images tend to become improper and further, an automatic toner sensor may become abnormal.
Further, in the HL type developing device it is very difficult to separate developer from the drawing up roller when compared with the separation of developer from the developing roller in a horizontal type developing device not the HL type developing device.
In other words, in case of an ordinary developing roller of a horizontal type developing device, the separation point having the weakest magnetic force between magnetic poles for separating developer is set below the horizontal line of the center of axis. Therefore, the separation itself is easy because of gravity of developer, etc. and the flow of separated developer is in the constant direction at the mixer side.
On the contrary, in case of the HL type developing device which draws up developer magnetically, the separation point is above the horizontal line passing through the center of axis and the separation itself is difficult. In addition, as the flow of separated developer is in the direction reverse to the mixer, the separation of developer is more difficult.
As described above, there is so far no HL type developing device that was able to perform the drawing up and separation of developer satisfactorily.